This invention relates to improvements which are primarily applicable to the structure and use of coordinate measuring probes which are used with coordinate measuring machines.
The head of a coordinate measuring machine is usually movable along three mutually perpendicular linear axes. A probe is mounted on the head, the head is moved to a position where the tip of the probe stylus contacts the workpiece which is being studied, and the x, y and z coordinates of tile head's position are sensed and recorded. A series of such readings are obtained and analyzed to obtain measurements of the workpiece.
A prevalent type of probe presently in use is a touch trigger probe. Such a probe provides a signal when it contacts the workpiece, and this signal triggers the taking of readings which indicate the head's position.
Another type of existing probe is a coordinate measuring probe. The probe disclosed in this specification is a new probe in this category. In a coordinate measuring probe, the probe has internal parts which are relatively movable in order to enable the stylus tip to move relative to the head of the coordinate measuring machine, along three mutually perpendicular axes. When such a probe is used, the displacement of the stylus tip relative to the head is is added to or subtracted from the head's coordinate location in the three coordinate directions. In some prior coordinate measuring probes, the displacement of the stylus tip from a neutral or zero position is sensed by linear variable differential transformers ("LVDT") and, in others, there are optical transducers with scales which are read optically by photosensitive pulse-counting-devices.
The structure and use of earlier coordinate measuring probes have presented several sources of error. For example, a lack of parallellism between the axes of the coordinate measuring machine and the coordinate measuring probe will lead to errors, as will nonlinear movement within the probe along any ostensibly linear axis. Friction between relatively movable parts of the probe presents another source of error. Metrology experts, recognizing these potential error sources, have endeavored to reduce such errors, but these efforts have focused primarily on improvement of the accuracy of the components of the measuring probes with respect to distance measuring and linearity.
LVDTs inherently produce errors because their output signals are not linear, they have a limited measurement range, they are slow to provide readings when a probe is moving in a dynamic mode, they introduce friction or require lubrication due to the sliding movement of a core within a tube, and their obligatory linear motion imposes restraints on the mechanical design of a probe.
Some aspects of the present invention are based upon the recognition that accuracy does not require precise linear movement between the relatively movable members of a coordinate measuring probe, nor does accuracy require parallelism between the coordinate axes of the probe and the coordinate axes of the machine head. Rather, it is possible to obtain a high degree of accuracy by using a mechanism which provides precise repeatability, and a calibration technique which creates a mathematical model which is capable of translating readings taken along the probe's coordinate axis system into readings which are expressed in terms of tile machine's cordinate axis system. Accuracy is also enhanced by a novel movement guiding device which assures precise repeatability, and by a novel optical position sensing device which operates between relatively movable members within the probe.
Although the principal applicability of the invention is in association with coordinate measuring machines, it is envisioned that the features disclosed in this specification will be useful in other areas where measurements are made, for example, in machine tools such as milling machines, jig borers and the like.